This application claims priority from Australian Provisional Patent Application No. 2005901352, the entire content of which is hereby incorporated by reference.
The most common mobile location systems at present are cell ID and enhanced cell ID systems. These systems use existing measurements within the mobile network. As a result they are able to locate existing handsets without requiring the modifications typically required by higher accuracy techniques.
The measurements available for use by such systems commonly include the identity of the serving cell, one or more round trip delays from a cell and signal levels measured by the mobile radio terminal (or mobile) from beacon channels broadcast by neighbouring cells.
Signal level measurements are useful in location calculation because received signal levels decrease with increasing range from the transmitter. Therefore, knowing the transmitted signal level and having measured the received signal level at the mobile, the attenuation or path loss provides an indication of the range between the transmitter and receiver.
One difficulty in practical systems is due to the fact that the attenuation is not a simple function of range. It is affected by a variety of factors including the specific frequency of the signal as well as the nature of the propagation channel. Relationships between path loss and range can be approximated by a closed form expression which yields useful results over some limited range of conditions. A difficulty with using signal levels in a specific location calculation remains however because such models are very general, useful more for system level modelling and radio network planning. Actual measurements in any particular location may deviate by a large margin.
One method that has been applied for using signal level measurements in mobile location is to perform a detailed survey and populate a database. This method however, suffers from the disadvantage of high cost and effort associated with populating the database. The area to be served by the system (for instance a city) has to be surveyed on a fine grid. Moreover if there are any significant changes in the actual path loss characteristics the survey has to be repeated. Such changes might arise from a change of season if there is a significant amount of foliage in the area. Other causes include the erection of a building or other edifice and maintenance of the mobile network.
In addition to the difficulties with obtaining a suitable model, further difficulties arise because the signal levels measured by a mobile are also affected by random (and uncorrelated with range) variations due to fading as well as measurement inaccuracies in the mobile. It is possible for the mobile to average multiple measurements in order to reduce the effects of noise and fast fading, however slow fading variations and biases due to the receiver remain.
These problems can be further exacerbated by user behaviour on the signal levels that are received. The way in which a user holds a mobile and also the way in which the mobile is positioned with respect to the head when in use can change the path loss by up to 10 dB. Furthermore, the user may put the terminal inside a briefcase or bag causing further signal loss. Further variations arise when a user enters a vehicle due to the shielding by the vehicle body. Yet further differences can arise depending on whether the phone is simply placed inside the vehicle or whether it is connected to a car kit with a roof or window mounted antenna. Again the differences in such cases can be as much as 10 dB. Still further variations occur when a mobile is operated inside a building compared to outdoors. All of these effects produce significant deviations in the path loss at a given range from what might be predicted using a general model of path loss versus range.
It is accordingly an object of the present invention to provide an improved path loss versus range model for a radio communications network, or to at least provide an alternative method of providing a path loss versus range model. The path loss versus range model may be used to calculate a location of a radio mobile terminal in the radio communications network.